Ruthenium-thioether Coordination Bonds for Photoresponsive Polymer Gels

Abstract

Smart materials are materials that can be manipulated to respond in a controlled fashion to external stimuli such as pH, temperature, light, moisture, stress, magnetic or electric fields. These materials are expected to play a significant role in the future because they are capable of introducing new functions and increasing the value of the materials. Photoresponsive materials are of particular interest. Light is a non-invasive and clean stimulus, it can be switch on and off spatiotemporally and remotely. In addition, the irradiation parameters can be easily tuned, thereby, allowing the precise manipulation of the photoreactions. Photoresponsive polymer gels have emerged as promising materials for diverse applications. In this thesis novel photoresponsive gel systems were prepared based on ruthenium(Ru)-thioether coordination bonds. In Chapter 2, we prepare a photo-controlled metallopolymer adhesive based on Ru-thioether coordination bonds for dynamic hydrogel assembly. Photoresponsive Ru-thioether coordination bonds as reversible crosslinks can control the adhesion between hydrogels. The adhesive is spread on the surfaces of hydrogels and is triggered to form a Ru-thioether coordination crosslinked polymer network entangled with the intrinsic polymer networks of hydrogels upon heating, inducing the formation of hydrogel assembly. Upon light irradiation, the Ru-thioether coordination crosslinked polymer network dissociates, resulting in photodetachment of the assembly. This adhesion approach allows for integrating various stimuli-responsive hydrogel building blocks into one assembly which shows several shape deformations under different stimuli. Furthermore, the hydrogel building blocks can be photodetached and reconfigured to generate other assemblies using the adhesive. In addition, combining a magnetical responsive hydrogel with other stimuli-responsive hydrogel assembly using adhesive endows the hydrogel assembly with mobility under a magnetic field. In Chapter 3, we describe a method to synthesize photoresponsive metallopolymer organohydrogels based on another kind of Ru-thioether coordination bond. Photoresponsive Ru-thioether coordination bonds in the polymer networks act as reversible crosslinks to control gel properties. A water/glycerol mixture is used as an anti-freezing solvent which can expand the usage of gel in cold environments. At -20 °C, the Ru-thioether coordination bonds dissociate upon visible light irradiation and reform in the dark. In this way the crosslink densities in the polymer network can be regulated by light, resulting in significant changes in the network structures and properties. This process enables inducing reversible gel-to-sol transitions, healing damaged gels, controlling the mechanical properties and volumes of the gels, and rewriting microstructures on the gels below 0 °C.